This invention relates to a closed loop control system to control the addition of a gas to a compressible mixture of liquid and gas. One aspect relates to a closed loop system that controls the addition of a gas to a compressible mixture of a liquid and such gas. An aspect of the invention relates to a closed loop control apparatus that traps a sample volume, then forces a plunger into the sample and measures the distance travelled by the plunger between two preset pressures of the sample, and then uses that measured distance to calculate a control signal. An aspect of the invention relates to a control system useful for adding a dry gas such as dry gaseous nitrogen to one of the liquid reagents in a recirculation reaction injection molding system for polyurethane molding in connection with which the invention will without prejudice be described.
In processing polyurethane materials, a foaming or blowing agent has been added to produce a cellular structure. The blowing agent may include water, Freon brand flourocarbon (commonly added as a liquid which vaporizes), and/or in some cases chemical reagents. It is desirable for ecological and cost reasons to eliminate the Freon if at all possible. Accordingly, the art has turned to dry gases, especially dry air or dry nitrogen. Sometimes these have been used in combination with Freon. The addition of gas bubbles in one of the reagents, is commonly referred to as nucleation in the reaction injection molding (RIM) process today. Normally, the nucleation is done in only one of the liquid reagents employed to form a polyurethane. Thus, while at one time gaseous or gas forming agents were used to make a foam of very light density, more recently engineered plastics structures such auto body parts and the like made of polyurethane and in some cases epoxies, utilize entrained and/or dissolved gases as a means to improve molding, extend the amount of plastic required, and even to reduce weight while not impairing mechanical properties significantly. Ranges of nucleation (gas bubble entrainment) in the monomer or other reagent are such that the bulk density or specific gravity of the mixture (i.e. liquid-gas bubble system) can be as low as 0.7 compared to 1.1 for the unnucleated liquid.
As faster reacting RIM chemical systems are developed, the need for accurate nucleation level control becomes more important.
One object of the present invention is to provide a method and apparatus to measure and control the nucleation level.
An object of the present invention is to provide a closed loop system to control nucleation level in response to a measure of the compressibility of the gas and liquid mixture.
The present invention thus provides a closed loop apparatus to control the flow of a gas into a liquid in automatic response to a determination or measurement of the gas in a sample of a mixture of said gas and said liquid comprising a recirculation system for said mixture having connected in series a source for said mixture, a place of use of said mixture, and a metering pump of the lance type which has a plunger reciprocable in a chamber; first, second and third conduit means connected, respectively, between the source and place, the place and pump, and the pump and source; means for adding gas to liquid connected into at least one of said recirculation system and conduit means; means to trap a sample of the mixture within the metering pump responsive to a trap signal to do so and to untrap such a sample responsive to another signal; means to determine the actual distance moved by the plunger in said chamber to change the sample pressure from a first preselected pressure to a second preselected pressure and to produce an actual distance signal representative of said actual distance; means to combine the actual distance signal with a set point signal to produce a control signal; means to control the gas flow to the means for adding gas in response to the control signal; and means to transmit the respective signals from where produced to where utilized.
Also, the present invention provides a process for closed loop control of the addition of a first fluid to a second or to a mixture of the aforementioned fluids to make a compressible mixture comprising the steps of providing, respectively, a trap chamber that includes a plunger reciprocable therein, means to add the first fluid into a fluid selected from the group consisting of said second fluid and a mixture of said first and second fluids to form said compressible mixture of fluids, and control means responsive to positioning of the plunger in the trap chamber for manipulating the means to add; trapping a sample of the mixture in the trap chamber; advancing the plunger into the chamber to change the sample pressure from a first preselected pressure to a second preselected pressure that is higher; producing an actual distance signal representative of the amount the plunger is advanced while changing the sample pressure as aforesaid; combining the actual distance signal with a set point signal to produce a control signal, the set point signal having been stored in the control means to produce a control signal representative of at least a portion of any difference between the signals so combined; and controlling the flow of said first fluid into said fluid selected from said group by applying said control signal to said control means.
The preferred embodiment of the present invention employs a gas as the compressible fluid and the mixture is preferably one of the polyurethane reagents in liquid form with some portion of such gas entrained. A dry gas is preferred, such as is dry nitrogen (N.sub.2) or dry air, preferably the nitrogen.